Nautilus depends on thumbnail source code files from the gnome-desktop project. Therefore, when the upstream https://gitlab.gnome.org/GNOME/gnome-desktop project releases a new version of the files, they should be downloaded and merged into the nautilus project.
The script downloads the latest gnome-desktop project files to the developer's local nautilus source folder so that the developer can commit the updated files to the nautilus project.
Fixes: #839